1. Field of the Invention
Exemplary embodiments of the present invention relate to a light-emitting diode and, more particularly, to a light-emitting diode having a distributed Bragg reflector.
2. Discussion of the Background
Gallium nitride (GaN)-based blue or ultraviolet (UV) light-emitting diodes (LEDs) may be used in a wide range of applications. In particular, various kinds of LED packages for emitting light having mixed colors, for example, white light, have been applied to backlight units, general lighting devices, and the like.
Since optical power of the LED package may depend upon luminous efficiency of an LED, numerous studies have focused on development of LEDs having improved luminous efficiency. For example, a metal reflector may be formed on a lower surface of a transparent substrate such as a sapphire substrate to improve light extraction efficiency of the LED.
FIG. 1 shows reflectivity of a sapphire substrate having an aluminum layer formed on a lower surface thereof.
Referring to FIG. 1, a sapphire substrate having no aluminum layer exhibits a reflectivity of about 20%, whereas the sapphire substrate having an aluminum layer exhibits a reflectivity of about 80% over the entire visible spectrum.
FIG. 2 shows reflectivity of a sapphire substrate having a distributed Bragg reflector formed by alternately stacking TiO2/SiO2 on a lower surface thereof.
When the substrate is formed with the distributed Bragg reflector instead of the aluminum layer, the substrate exhibits a reflectivity approaching 100% for light in the blue wavelength range, for example in a wavelength range of 400 nm to 500 nm and having a peak wavelength of 460 nm, as shown in FIG. 2.
However, the distributed Bragg reflector may only increase reflectivity in certain regions of the visible spectrum and may exhibit significantly lower reflectivity in other regions. In other words, as shown in FIG. 2, the reflectivity rapidly decreases at a wavelength of about 520 nm or more and is less than 50% at a wavelength of 550 nm or more.
Accordingly, when an LED with the distributed Bragg reflector is mounted on an LED package for emitting white light, the distributed Bragg reflector of the LED may exhibit high reflectivity with respect to light in the wavelength range of blue light emitted from the LED, but may not exhibit effective reflective characteristics with respect to light in the wavelength ranges of green and/or red light, thereby restricting improvement in light emission efficiency of the LED package.
A GaN-based semiconductor has an index of refraction of about 2.4. Accordingly, there may be a difference in index of refraction between the GaN-based semiconductor and external air or a molding resin, so light generated in the active layer may be trapped by the semiconductor layer and not be emitted to the outside due to total internal reflection at an interface therebetween.